This invention relates to the electrodeposition of zinc, and particularly to a plating bath for plating bright, level zinc deposits from aqueous acid plating baths. More particularly, the improvement relates to incorporation in the bath of a phosphorus cation having the formula: EQU R.sub.4 P.sup. .sym.
wherein each R is independently a hydroxy alkyl group. The invention also relates to methods for electrodeposition of level zinc deposits from such baths and to additive compositions for preparing and maintaining the baths.
Considerable attention has been directed to the development of zinc electroplating baths which will produce bright and level zinc deposits of improved quality. Research has been devoted to improving the overall brightness, the range of allowable current densities, and the ductility of the zinc deposit with alkaline zinc baths dominating the field of zinc electroplating for many years due primarily to the heretofore unsolved problems accompanying the use of acid zinc baths specified above. Until recently, most of the successful aqueous alkaline zinc plating baths have contained substantial quantities of cyanide which has caused concern regarding toxicity and waste disposal problems.
The enactment and enforcement of the various environmental protection laws, especially those designed to improve water quality, have increased the pressures on the industry to reduce or eliminate the use and discharge of baths containing cyanides. As a result, increased efforts to develop bright zinc plating processes to replace the classical zinc cyanide baths has been increased.
Typically, acid plating baths have been based on a suitable inorganic zinc salt such as zinc sulfate, zinc cloride, zinc fluoborate, zinc acetate, zinc sulfamate, or zinc pyrophosphate. The bath usually includes a buffer such as the corresponding ammonium salt and other additives to promote and improve ductility, brightness, throwing power and covering power. Surface active agents may be included to improve the crystal structure, reduce pitting, and increase the solubility of the other additives.